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Syrian defector speaks

Just Watched

Syrian writer was beaten, humiliated

The UK Foreign Office did not want to comment directly on the arrests but said the "deteriorating security situation (in Syria) leaves dangerous space for foreign fighters."

"We are concerned about reports of extremist groups operating in Syria and we take very seriously any claims that British nationals are among foreign fighters there," the Foreign Office statement added.

The two people arrested at Heathrow are both British nationals, according to police sources.

One line of inquiry being followed by investigators, the police sources said, is whether the two suspects were involved in an incident in July in which Sunday Times photographer John Cantlie said he was held at gunpoint by rebels in Syria.

Some of those who detained him spoke with an English accent, Cantlie said.

The photographer declined to comment on the arrests or the incident in Syria when contacted by CNN Wednesday.